These are unused sprites found in the game’s files. Here’s a handful of them.
The sprites are called “chara”, with the last letter indicating the direction they are facing (l - left and r - right). There are sprites for up and down as well. These can only be seen in debug mode, specifically in Frisk’s reflection in the room full of puddles.
The purpose for the “chara’ sprite is not certain – perhaps it was meant to hint at Chara’s presence throughout the game, or foreshadow their potential take over in the genocide route.
Notably, the sprites for Frisk are called “mainchara”, while those for Chara are called “truechara.”